Onium salts such as iodonium and sulfonium salts have been heretofore known as cationic polymerization initiators for curing a cationic polymerizable compound such as an epoxy compound by application of heat, light or an active energy ray such as an electron beam (see Patent Documents 1 to 10).
In addition, since such an onium salt generates an acid by application of heat or an active energy ray, the onium salt is also referred to as an acid generator, and also used for resists and photosensitive materials (Patent Documents 11 to 13).
Incidentally, although cationic polymerization initiators (acid generators) described in these specifications contain BF4−, PF6−, AsF6−, or SbF6− as an anion, curing performance of cationic polymerizable compound and crosslinking reaction performance varies with the kind of an anion and these performances are improved in an ascending order of BF4−<PF6−<AsF6−<SbF6−. However, applications of cationic polymerization initiators (acid generators) containing AsF6− or SbF6− which have good polymerization performance and crosslinking reaction performance are limited from a toxicity problem of As or Sb, and only an SbF6− salt is used in a limited field such as photofabrication. On that account, although a PF6− salt being poor in the polymerization performance and crosslinking reaction performance by acid catalyst is generally utilized, for example, in order to attain a curing rate almost comparable to that of an SbF6− salt, since the PF6− salt in an amount being nearly ten times the amount of the SbF6− salt needs to be added and the remaining amount of an unreacted initiator (acid generator), a solvent used as necessary for dissolving the initiator (acid generator), or a decomposition product of the initiator is increased, there are problems that physical properties of a cured material are impaired, and moreover, a base material, facilities, and the like are liable to be corroded because of an increased amount of HF produced as a by-product by decomposition of the initiator. As such, a cationic polymerization initiator that is free from a toxic metal and has a cationic polymerization-initiating ability comparable to that of an SbF6− salt has been strongly required.
In members that are required to have optical properties, such as displays, optical waveguides and optical lenses, importance is placed on transparency of cured products cured by application of heat, light or irradiation with an active energy ray such as an electron beam, and transparency of cured products after a heat resistance test and after a humidity resistance test. In addition, applications requiring corrosion resistance to a remaining strong acid include members such as paint compositions, coating agents, ink compositions, inkjet ink compositions, resist films, liquid resists, negative resists, MEMS resists, negative photosensitive materials, various adhesives, molding materials, casting materials, putty materials, glass fiber impregnating agents, fillers, sealing agents, sealants, optical semiconductor (LED) sealants, optical waveguide materials, nano-imprint materials, stereolithography materials, micro-stereolithography materials, and ACF (anisotropic conductive films).
The present inventors have proposed a fluorinated alkylphosphonic acid onium salt-based acid generator (Patent Document 14) as a cationic polymerization initiator (acid generator) which does not contain a toxic metal, and has cationic polymerization performance and crosslinking reaction performance comparable to those of a SbF6− salt. However, a cured product including the acid generator has the problem that transparency is deteriorated particularly after a heat resistance test, and application of the acid generator to the above-mentioned members that are required to have optical properties has not progressed.
In addition, an onium salt having tetrakis(pentafluorophenyl)borate as an anion (Patent Document 15) is known as a cationic polymerization initiator (acid generator) which does not contain a toxic metal, and has cationic polymerization performance and crosslinking reaction performance comparable to those of a SbF6− salt. However, a cured product including the acid generator has the problem that transparency is deteriorated because HB(C6F5)4 as a strong acid remaining particularly after a heat resistance test causes corrosion and coloring of a resin etc., and application of the acid generator to the above-mentioned members that are required to have optical properties has not progressed.